Slow wave meander line loaded antennas are known, with the meander line providing for a narrow band and a wide band response, depending on the application. One patent describing such a slow wave meander line structure is U.S. Pat. No. 6,313,716 assigned to the assignee hereof and incorporated herein by reference. In this meander line embodiment, the meander line includes an electrically conductive plate, and a plurality of transmission line sections supported with respect to the conductive plate. The plurality of sections includes a first section loaded relatively closer and parallel to the conductive plate to have a relatively lower characteristic impedance with the conductive plate, and a second section located parallel to and at a relatively greater distance from the conductive plate than the first section to have a relatively higher characteristic impedance with the conductive plate. A conductor is provided for interconnecting the first and second sections and maintaining an impedance mismatch therebetween.
These meander line structures can utilized either as antennas themselves or as coupling devices to antennas such as described in Provisional Patent 60/435,099 filed Dec. 20, 2002 by John T. Apostolos entitled VITL Based Universal Antenna Coupler. The largest problem with such meander line structures is their low frequency cutoff. While meander lines are used to provide a compact or miniaturized device no matter what the frequency band, for each band obtaining a lower frequency cutoff is important.
For instance, for low frequency communication in which a grounded loop antenna replaces the traditional whip antenna mounted to a vehicle, the ability to operate down to 4 MHz is vitally important. The low frequency requirement is to assure close-in sky wave communications by having the take-off angle as steep as possible. However, getting a miniaturized coupler to operate at 4 MHz is a problem. Either meander line couplers have to double their footprint over what is acceptable or the antenna has to be elongated and may extend up too far, meaning it can get caught on trees or overhanging vegetation, to say nothing of low lying power or telephone lines.
Moreover, meander line couplers can have various meander line sections switched in and out to change the frequency at which the meander line is tuned. Because the PIN diode or FET switches are placed between a high impedance section of the meander line and a low impedance section, the open switch differential voltage across the switch may be in excess of 10,000 volts. This causes substantial voltage stress that can cause the switches to fail, which in turn limits the transmit power allowed so as not to burn out the switches. While in a tactical situation one might want to switch from 100 watts to 300 watts, switch failure would prevent one from so doing.
Going from military to civilian use, for the cellular and PCS bands it is important to provide a miniature wide band antenna that can operate between 800 and 3,000 MHz. Unfortunately it is only with difficulty that one can get below 1500 MHz using standard meander line loaded antennas. In short, for standard meander line loaded antennas there is a severe low frequency threshold. This limits how low a cutoff frequency for the meander line can be. What is needed is a breakthrough in the low frequency cutoff of meander line loaded antennas for such applications.
A third application is for military communications in the 30–88 MHz band. What is required is a reduced footprint antenna that is small enough to be carried on a vehicle or aircraft and yet operate in the 30–88 MHz band. Standard meander line loaded antennas, while small, are nonetheless too large at 30 MHz. Again, what is needed is a breakthrough in the lowering of the low frequency cutoff for meander line structures in the 30–88 MHz range so that a suitably sized device will work.
Whether it be for 4 MHz communications, 30 MHz communications or 800+ MHz communications, there is a need for a compact device having a reduced the low frequency cutoff. Note that a standard meander line coupler at 4 MHz would have a footprint of 28″×50″, too large to be placed on the top of a small vehicle. For the 30–88 MHz range a meander line loaded antenna would have to be as large as 16″×48″×48″, too large for vehicle or aircraft use. In the cellular and PCS applications, meander line loaded antennas are only 0.3″ high×1.2″ wide×1.2″ long. However, their low frequency cutoff is approximately 1500 MHz, too far above the cellular 800 MHz band.
What is therefore necessary is a new meander line configuration to dramatically lower the low frequency cutoff of such devices.
By way of further background, for military use, taking a tactical situation in which a soldier or vehicle needs to communicate with another soldier or vehicle at some distance away, typically communications is provided through the use of a ground wave and also from skip off the ionospheric layer. While a ground wave is usually viable up to about 30 miles from the transmission site, if the skip angle is shallow, there will be a significant blackout or dead zone along the ground, say from 30 miles to 100 miles, where there will be no communications possible. This is because the transmitted radiation skips over this ground segment before it is reflected down to the surface of the earth.
When depending on a sky wave or a skip for robust communications, the takeoff angle of the radiation is indeed important. It is noted that the higher the frequency the more shallow is the takeoff angle such that there is more of an extended dead zone which starts at the transmission site and extends to the point at which radiation reflected from the ionosphere strikes the surface of the earth. This means that there is a communications blackout zone, for instance, between 30 miles and 100 miles when a transmitter is operating in the 5 MHz frequency band. This is because of the somewhat shallow takeoff angle in which no radiation from the transmitter reaches a position on the surface of the earth beyond the point at which the ground wave dissipates. Thus in the above example, there would be no communication possible between 30 miles and 100 miles from the transmitter.
Where it possible to be able to lower the operating frequency of the transmitter to, for instance, 4 MHz, then the takeoff angle would be higher and radiation returned from the ionosphere would be closer to the transmitter, e.g. between 30–100 miles of the transmitter. What this means is that communications could established from the transmitter all the way up to the 30 mile limit of the ground wave transmission and then up to another 100 miles due to the sharper skip angle involved with operating at the lower frequency.
While it is certainly possible with a long whip antenna to be able to transmit at 4 MHz, it would be desirable to be able to use a short radiator and a meander line structure as a miniature coupler to permit operating at 4 MHz. Thus, rather than having to have a quarter wave antenna at 4 MHz, one needs to find how to construct a miniaturized coupler for a very short length whip or loop. One therefore needs to develop a meander line coupling device that without enlarging the device would lower the VSWR to less than 2:1 at the lower frequency. This would permit a continuum in the communications capabilities of the transmitter while at the same time using a smaller radiator and the same miniaturized meander line coupler.
For 30–88 MHz use, this is a frequency hopping communications band used extensively by the military. The antenna structures for this band are sizeable and there is a need to be able to reduce the size of the antenna structures so that they can be readily mounted to vehicles or aircraft. While meander line couplers and antennas have been proposed for such use, they cannot be made to operate close to 30 MHz, at least at sizes that are required. To make such an antenna operate at 30 MHz the size required is a volume 16″ high×48″ wide×48″ long, or 36,864 cubic inches. This resulted in rejection of such antennas for tanks and some aircraft. If one could design a wideband antenna for this band at 10″×32″×32″ or 10,240 cubic inches, then there is enough real estate on the vehicle due to a volume reduction of 3.7:1.
Another antenna related problem is one that is typical of cell phone antennas. First, one needs a compact wideband antenna that can cover the cellular band at 800 MHz, and the PCS bands at 1.7–1.9 GHz, as well as operating at the GPS frequency of 1.575 GHz. Getting a meander line loaded antenna to operate down to 800 MHz at the current size required is a challenge.
Moreover, there is another problem that needs to be resolved with wideband cellular antennas. Since most cell phone antennas are backed with a ground plane, usually the ground plane of the printed circuit board within the cell phone, there is a problem called “down firing”, in which the major lobe of the antenna points into the ground. This limits the ability of the hand held device both in the receive and in the transmit mode because radiation transmitted from such a device is fired into the ground, whereas the receive characteristic is diminished in the horizontal direction. While meander line loaded antennas have been used in cell phones because of their small size and wide bandwidth operating in the 800 MHz, 1.7 GHz and 1.9 GHz bands, they nonetheless suffer from “down firing” at frequencies above 1.7 GHz. It would be convenient if some meander line structure could also eliminate the down firing problem.